The invention relates to photorefractive compositions. More particularly, the invention relates to copolymers that include functional groups which provide photorefractive capabilities.
Photorefractivity is a phenomenon in which the refractive index of a material can be altered by changing the electric field within the material, such as by laser beam irradiation. The change of the refractive index is achieved by a series of steps, including: (1) charge generation by laser irradiation, (2) charge transport, resulting in the separation of positive and negative charges, and (3) trapping of one type of charge (charge delocalization), (4) formation of a non-uniform internal electric field (space-charge field) as a result of charge delocalization, and (5) refractive index change induced by the non-uniform electric field.
Therefore, good photorefractive properties can be seen only for materials that combine good charge generation, good charge transport or photoconductivity, and good electro-optical activity.
Photorefractive materials have many promising applications, such as high-density optical data storage, dynamic holography, optical image processing, phase conjugated mirrors, optical computing, parallel optical logic, and pattern recognition.
Originally, the photorefractive effect was found in a variety of inorganic electro-optical (EO) crystals, such as LiNbO3. In these materials, the mechanism of the refractive index modulation by the internal space-charge field is based on a linear electro-optical effect.
In 1990 and 1991, the first organic photorefractive crystal and polymeric photorefractive materials were discovered and reported. Such materials are disclosed, for example, in U.S. Pat. No. 5,064,264, to Ducharme et al. Organic photorefractive materials offer many advantages over the original inorganic photorefractive crystals, such as large optical nonlinearities, low dielectric constants, low cost, lightweight, structural flexibility, and ease of device fabrication. Other important characteristics that may be desirable depending on the application include sufficiently long shelf life, optical quality, and thermal stability. These kinds of active organic polymers are emerging as key materials for advanced information and telecommunication technology.
In recent years, efforts have been made to optimize the properties of organic, and particularly polymeric, photorefractive materials. As mentioned above, good photorefractive properties depend upon good charge generation, good charge transport, also known as photoconductivity, and good electro-optical activity. Various studies that examine the selection and combination of the components that give rise to each of these features have been done. The photoconductive capability is frequently provided by incorporating materials containing carbazole groups. Phenyl amine groups can also be used for the charge transport part of the material.
Non-linear optical ability is generally provided by including chromophore compounds, such as an azo-type dye, which can absorb photon radiation. The chromophore may also provide adequate charge generation. Alternatively, a material known as a sensitizer may be added to provide or boost the mobile charge required for photorefractivity to occur. Many materials, including a wide range of dyes and pigments, can serve as sensitizers.
The photorefractive composition may be made simply by mixing the molecular components that provide the individual properties required into a host polymer matrix. However, most compositions prepared in this way are not stable over time, because phase separation tends to occur as the components crystallize.
Efforts have been made, therefore, to make polymers that include one or more of the active components in the polymer structure.
An example of a polymer matrix that includes transport components is poly(n-vinylcarbazole) (PVK). With such a matrix, polymers with: high performance could be fabricated as reported by N. Peyghambarian et al. (Nature, 1994, 371, 497).
In this case, a, photorefractive composition was made by adding an a{dot over (z)}o dye (DMNPAA; 2,5-dimethyl-4-(p-nitrophenylazo) anisole) as chromophore, and trinitrofluorenone (TNF) as sensitizer. The resulting compositions showed almost 100% diffraction efficiency at laser intensity of 1W/cm2 and 90 V/xcexcm biased voltage. However, the response time was slow at over 100 msec.
To achieve good photorefractivity, however, such materials must be doped with large concentrations of chromophore, such as 25 wt % or more. Thus, crystallization and phase separation of the strongly dipolar chromophore remain a major problem.
To completely eliminate the instability caused by phase separation, it has been recognized that it would be desirable to prepare fully functionalised photorefractive polymers, that is, polymers in which both the photoconductivity and the non-minear optical capability reside within the polymer itself.
Building on the original University of Arizona work, efforts have been made to develop fully functional photorefractive polymers, as well as to speed up the response time. For example, PVK polymers in which some of the carbazole groups are tricyanovinylated have been made (N. Peyghambarian et al., Applied Phys. Lett., 1992, 60, 1803). However, the photoconductivity of this polymer was reported as only 0.98 pS/cm and the diffraction efficiency was less than 1%, too low to show good photorefractivity. Subsequently, the same group has reported PVK-based materials with an amazing response time of 4 msec. (N. Peyghambarian et al., Applied Physics Leiters, 1999, 16, 2253).
A number of efforts at materials improvement have used methacrylate-based polymers and copolymers that include photoconductive and chromophore side groups. A paper by T. Kawakami and N. Sonoda, (Applied Phys. Lett., 1993, 62, 2167.) discloses acrylate-based polymers containing dicyanovinylideneyl phenylamines as charge transport groups. The diffraction efficiency was reported at around 0.01%.
Japanese Patent Application Laid-open JP-A-1995-318992, to Hitachi Ltd. discloses acrylate-based polymers and copolymers made by conventional polymerization techniques and containing charge transport and non-linear-optical groups, but gives no photorefractive performance data.
A report by H. Sato et al., (Technical report of IEICE., 1995, OME-95-53, OPE95-94, 43) describes the preparation of several copolymers having both charge transport components and non-linear optical components in the side groups of the copolymer. However, the charge transport speeds seem to be too slow for good photorefractive materials.
Japanese Patent Application Laid-open JP-A 1998-333195, to Showa Denko, discloses acrylate-based polymers incorporating triphenylamine groups as charge transport agents. Fast response times (50 msec. at 70 V/xcexcm biased voltage) is reported, although there is no description or data regarding diffraction efficiency.
A paper by Van Steenwickel et al. (Macromolecules, 2000, 33, 4074) describes acrylate-based polymers that include carbazole-based side chains and several stilbene-type side chains. The paper cites a high diffraction efficiency of 60% at 58 V/xcexcm, but a slow-response time of the sub-second order.
A paper by Y.Chen et al. (Modern Optics, 1999, 46, 1003) discusses a methacrylate polymer that has both carbazole-type side chains to provide charge transport capability and nitrophenyl azo-type side chains to provide non-linear optical capability. The materials again show slow response times of over 20 sec.
None of the materials described above achieves the optimum combination of a high diffraction efficiency with a fast response time, long-term stability and easy processability. Thus, there remains a need for photorefractive compositions that combine these attributes.
In recent years, a new type of polymerization, termed living radical polymerization, has been developed for polymerization of functional monomers, including methacrylate and styrene derivatives. Living radical polymerization differs from conventional radical polymerization in that the polymer growth terminals can be temporarily protected by protection bonding. This enables polymerization to be well controlled, including being stopped and started at will.
This process can be used to prepare homopolymers and copolymers, including block copolymers. Details of the living radical polymerization method are described in prior literature. They may be found, for example, in the following papers:
1. T. Patten et al., xe2x80x9cRadical polymerization yielding polymers with Mw/Mnxcx9c1.05 by homogeneous atom transfer radical polymerizationxe2x80x9d, Polymer Preprints, 1996, 37, 575.
2. Matyjasewski et al., xe2x80x9cControlled/living radical polymerization. Halogen atom transfer radical polymerization promoted by a Cu(I)/Cu(II) redox processxe2x80x9d, Macromolecules, 1995, 28, 7901.
3. M. Sawamoto et al., xe2x80x9cRuthenium-mediated living radical polymerization of methyl methacrylatexe2x80x9d, Macromolecules, 1996, 29, 1070.
Living radical polymerization is also described in U.S. Pat. No. 5,763,548 to Carnegie-Mellon University, which is incorporated herein by reference in its entirety.
As an example of block copolymers prepared by living radical polymerization, novel styrene and butyl acrylate block copolymers for pressure sensitive adhesives have been reported (JP-A 2001-115124, M. Yamamoto et al.). Such block copolymers could not be prepared by conventional polymerization methods.
The object of the present invention is to provide a photorefractive composition which exhibits high photoconductivity, and a copolymer which is desirably used for the photorefractive composition.
A first aspect of the present invention is a composition comprising a co-polymer, wherein: (a) the co-polymer comprises a first repeating unit including a first moiety selected from the group consisting of the structures (i), (ii) and (iii), and a second repeating unit including a second moiety represented by the formula (0); wherein (b) the composition exhibits photorefractive ability: 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is between about 2 and 6; and wherein Ra1, Ra2, Ra3, Ra4, Ra5, Ra6, Ra7, and Ra8 are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is between about 2 and 6; and wherein Rb1-Rb27 are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is between about 2 and 6, and wherein Rc1-Rc14 are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; 
wherein Q represents an alkylene group, with or without a hetero atom; R1 is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; G is a group having a bridge of xcfx80-conjugated bond; and Eacpt is an electron acceptor group.
A second aspect of the present invention is a composition comprising a co-polymer prepared by radical polymerization, wherein: (a) the co-polymer comprises a first repeating unit selected from the group consisting of the structures (i)xe2x80x3, (ii)xe2x80x3 and (iii)xe2x80x3, and a second repeating unit represented by the formula (0)xe2x80x3; wherein (b) the composition exhibits photorefractive ability: 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is between about 2 and 6; and wherein Ra1, Ra2, Ra3, Ra4, Ra5, Ra6, Ra7, and Ra8 are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is between about 2 and 6; and wherein Rb1-Rb27 are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is between about 2 and 6, and wherein Rc1-Rc14 are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons; and an aromatic group with up to 10 carbons; 
wherein Q represents an alkylene group, with or without a hetero atom; R1 is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; G is a group having a bridge of xcfx80-conjugated bond; and Eacpt is an electron acceptor group.
A third aspect of the present invention is a composition comprising a block co-polymer, wherein: (a) the block co-polymer comprises a first block unit including a first moiety selected from the group consisting of the above structures (i), (ii) and (iii), and a second block unit including a second moiety represented by the above formula (0); wherein (b) the composition exhibits photorefractive ability:
A fourth aspect of the present invention is a composition comprising a block co-polymer prepared by living radical, polymerization, wherein: (a) the block co-polymer comprises a first block unit having charge transport ability, and a second block unit having non-linear-optical ability; wherein (b) the composition exhibits photorefractive ability.
A fifth aspect of the present invention is a composition comprising a block co-polymer, wherein: (a) the block co-polymer comprises a first block unit containing a unit selected from the group consisting of the above structures (i)xe2x80x3, (ii)xe2x80x3 and (iii)xe2x80x3, and a second block unit containing a unit represented by the above formula (0)xe2x80x3; wherein (b) the composition exhibits photorefractive ability.
The composition comprises a polymer matrix, and includes a component that provides photoconductive (charge transport) ability and a component that provides non-linear optical ability, and, optionally, sensitizer and plasticizer components. Both of the photoconductive and non-linear optical components are incorporated into the chemical structure of the polymer itself, typically as side groups.
The composition differs from photorefractive compositions previously known in the art in several points.
In a first point, the composition provides fast response time compared with prior art photoconductive materials; and/or one or more other advantageous properties, such as high diffraction efficiency and high photoconductivity. Furthermore these properties can typically be provided in conjunction with one or more other desirable attributes, such as, comparatively low glass transition temperature, low viscosity, and excellent handling and processing capability.
In a second point, the composition comprises a random copolymer or a block copolymer incorporating blocks containing the photoconductive side group and blocks containing the chromophore side group. Random-type or block-type copolymers comprising two functional groups, a charge transport functional group and a non-linear optical functional group, are provided, and can be readily made by the processes of the invention.
In a third point, the composition comprises a copolymer which showed very good phase stability, that is, resistance to phase separation.
Both the random copolymers and the block copolymers disclosed herein provide the advantage of long-term stability, due to lower likelihood of phase separation or crystallization, compared with polymer materials in which the functionality is provided by adding functional materials in the form of dopants.
With respect to the first point of the invention, inventors have found, to inventors"" surprise, that inventors"" photorefractive compositions exhibit high response times, such as 50 ms or less.
With respect to the second point of the invention, inventors have developed photorefractive copolymers in the art. The copolymers of the invention can manifest essentially any combination of units having photoconductive ability and units having non-linear optical ability.
The copolymers of the invention can be readily made by adapted conventional radical polymerization techniques, such as by azo-type initiator system catalysis or by the adapted living radical polymerization techniques.
The block copolymers of the invention can manifest essentially any combination of blocks containing units with photoconductive ability and blocks containing units with non-linear optical ability. For example, if A represents a polymer block that incorporates charge transport ability groups and B represents a polymer block that incorporates non-linear-optical ability groups, then the copolymers of the invention include any combinations of A and B units. As representative, but non-limiting examples, polymers of the forms A-B, B-A, A-B-A, B-A-B, and so on, are included.
With respect to the third point of the invention, inventors have developed photorefractive copolymers which are composed of a component that provides photoconductive (charge transport) ability and a component that provides non-linear optical ability. Since the copolymers have both components of charge transport and non-linear optical ability in one polymer chain, it can work as a kind of phase compatibilizer for all components to be mixable without separations. Even if the chtomophore or plascticizer functional material is mixed in the form of dopant, the composition still provides the long-term stability.
The comparatively lower Tg of inventors"" polymers reduces the dependence on large amounts of plasticizer in the finished composition. This improves the handling properties and the photorefractive properties of the composition.
The photorefractive compositions according to the present invention have great utility in a variety of optical applications, including holographic storage, optical correlation, phase conjugation, non-destructive evaluation and imaging.
The invention is a photorefractive composition. The composition comprises a copolymer matrix that provides photoconductive (charge transport) ability and a non-linear optical ability. Also, the copolymer may also comprise a part that provides a plasticizer ability. Optionally, the composition may also include other components as desired, such as sensitizer and plasticizer components.
Both of the photoconductive and non-linear optical components are incorporated as functional groups into the polymer structure, typically as side groups.
The group that provides the charge transport or photoconductive functionality may be any group known in the art to provide such capability. If this group is to be attached to the polymer matrix as a side chain, then the group should be capable of incorporation into a monomer that can be polymerized to form the polymer matrix of the composition.
Preferred photoconductive groups are phenyl amine derivatives, particularly carbazoles and di- and tri-phenyl diamine.
Most preferably the moiety that provides the photoconductive functionality is chosen from the group of phenyl amine derivatives consisting of the following structures (i) to (iii): 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is between about 2 and 6; and wherein Ra1, Ra2, Ra3, Ra4, Ra5, Ra6, Ra7, and Ra8 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is between about 2 and 6; and wherein Rb1-Rb27 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; and 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is between about 2 and 6, and wherein Rc1-Rc14 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group, with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons.
Likewise, the chromophore or group that provides the non-linear optical functionality may be any group known in the art to provide such capability. If this group is to be attached to the polymer matrix as a side chain, then the group, or a precursor of the group, should be capable of incorporation into a monomer that can be polymerized to form the copolymer matrix of the composition.
Preferred chromophore groups are aniline-type groups or dehydronaphtyl amine groups.
The chromophore or group that provides the non-linear optical functionality used in the present invention is represented by formula (0): 
wherein Q represents an alkylene group, with or without a hetero atom; such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is an integer of about 2 to 6; R1 is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons, and preferably R1 is an alkyl group which is selected from methyl, ethyl, propyl, butyl, pentyl and hexyl; G is a group having a bridge of xcfx80-conjugated bond; and Eacpt is an electron acceptor group.
In the above definition, by the term xe2x80x9ca bridge of xcfx80-conjugated bondxe2x80x9d, it is meant a molecular fragment that connects two or more chemical groups by xcfx80-conjugated bond. A xcfx80-conjugated bond contains covalent bonds between atoms that have "sgr" bonds and xcfx80 bonds formed between two atoms by overlap of their atomic orbitals (s+p hybrid atomic orbitals for a bonds; p atomic orbitals for xcfx80 bonds).
By the term xe2x80x9celectron acceptorxe2x80x9d, it is meant a group of atoms with a high electron affinity that can be bonded to a xcfx80-conjugated bridge. Exemplary acceptors, in order of increasing strength, are:
C(O)NR2 less than C(O)NHR less than C(O)NH2 less than C(O)OR less than C(O)OH less than C(O)R less than C(O)H less than CN less than S(O)2R less than NO2 
As typical exemplary electron acceptor groups, functional groups which is described in prior of art U.S. Pat. No. 6,267,913 and shown in the following structure figure can be used. U.S. Pat. No. 6,267,913 is hereby incorporated by reference for the purpose of describing donors and acceptors useful in this invention. The symbol xe2x80x9c‡xe2x80x9d in a chemical structure herein specifies an atom of attachment to another chemical group and indicates that the structure is missing a hydrogen that would normally be implied by the structure in the absence of the xe2x80x9c‡xe2x80x9d. 
wherein R is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 atoms, a branched alkyl group with up to 10 atoms, and an aromatic group with up to 10 carbons.
Most preferably the moiety that provides the non-linear optical functionality is such a case that G in formula (0) is represented by a structure selected from the group consisting of the structures (iv), (v) and (vi); 
wherein, in both structures (iv) and (v), Rd1-Rd4 are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 atoms, a branched alkyl group with up to 10 atoms, and an aromatic group with up to 10 carbons, and preferably Rd1-Rd4 are all hydrogen; R2 is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 atoms, a branched alkyl group with up to 10 atoms, and an aromatic group with up to 10 carbons; 
wherein R7 represents a linear or branched alkyl group with up to 10 carbons; and
wherein Eacpt in formula (0) is an electron acceptor group and represented by a structure selected from the group consisting of the structures; 
wherein R9, R10, R11, and R12 are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 atoms, a branched alkyl group with up to 10 atoms, and an aromatic group with up to 10 carbons.
A preferred copolymer used for the photorefractive composition is a co-polymer which comprises a first repeating unit selected from the group consisting of the structures (i)xe2x80x3, (ii)xe2x80x3 and (iii)xe2x80x3, and a second repeating unit represented by the formula (0)xe2x80x3. 
wherein Q and Ra1-Ra8 are the same meaning as in Structure (i); 
wherein Q and Rb1-Rb27 are the same meaning as in Structure (ii); 
wherein Q and Rc1-Rc14 are the same meaning as in Structure (iii); 
wherein Q, R1, G and Eacpt are the same meaning as in formula (0).
The copolymer matrix is preferably synthesized from a monomer incorporating both one of the above photoconductive groups and one of the above chromophore groups. The inventors have recognized that a number of physical and chemical properties are desirable in the copolymer matrix. It is preferred if the copolymer itself incorporates both a charge transport group and a chromophore group, to form copolymers. Physical properties of the formed copolymer that are of importance are the molecular weight and the glass transition temperature, Tg. Also, it is valuable and desirable, although not essential, that the copolymer should be capable of being formed into films, coatings and shaped bodies of various kinds by standard polymer processing techniques, such as solvent coating, injection molding and extrusion.
In the present invention, the copolymer generally has a weight average molecular weight, Mw, of from about 3,000 to 500,000, preferably from about 5,000 to 100,000. The term xe2x80x9cweight average molecular weightxe2x80x9d as used herein means the value determined by the GPC (gel permeation chromatography) method in polystyrene standards, as is well known in the art.
For good photorefractive properties, the photorefractive composition should be substantially amorphous and non-crystalline or non-glassy under the conditions of use. Therefore, it is preferred that the finished photorefractive composition has a relatively low glass transition temperature, Tg, such as below about 50xc2x0 C., more preferably below about 40xc2x0 C. Preferred temperature ranges for the Tg are 10-50xc2x0 C., most preferably 20-40xc2x0 C. If the pure polymer itself has a glass transition temperature higher than these preferred values, which will generally be the case, components may be added to lower the Tg, as discussed in more detail below.
Nevertheless, it is preferred that the polymer itself has a relatively low glass transition temperature, by which inventors mean a Tg no higher than about 125xc2x0 C., more preferably no higher than about 120xc2x0 C., and most preferably no higher than about 110xc2x0 C. or 100xc2x0 C.
Particularly, in order to lower glass transition temperature of copolymer itself, the already described monomers that provides the plasticizing ability can be incorporated. Sometimes the incorporation of plasticizing monomers into the copolymer which has charge transport and non-linear-optics ability can reduce the glass transition temperature more than 50xc2x0 C. or 20xc2x0 C., at least 5xc2x0 C., depending on incorporation ratio.
In more detail, the moiety that provides the plasticizing ability is chosen from the group consisting of a linear alkyl group with up to 10 carbons, a linear alkenyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and a branched alkenyl group with up to 10 carbons. In more detail, the monomer is chosen from the group of CH2xe2x95x90CR0xe2x80x94COOR wherein R0 represents a hydrogen atom or methyl group, and R represents a C2-10 alkyl group, such as butylacrylate, ethyl acrylate, propyl acrylate, 2-ethylhexyl(meth)acrylate and hexyl(meth)acrylate.
A relatively low glass transition temperature is preferred because a greater mobility of polymer chains that exhibit close to or above their glass transition temperature give higher orientation during voltage application, and lead to better performance, such as high photoconductivity, fast response time and high diffraction efficiency, of the photorefractive device.
In principle, essentially any polymer backbone, including, but not limited to, polyurethane, epoxy polymers, polystyrene, polyether, polyester, polyamide, polyimide, polysiloxane, and polyacrylate could be used, with the appropriate side chains attached, to make the polymer matrices of the invention.
Preferred types of backbone units are those based on acrylates or styrene. Particularly preferred are acrylate-based monomers, and more preferred are methacrylate monomers. The first polymeric materials to include photoconductive functionality in the polymer itself were the polyvinyl carbazole materials developed at the University of Arizona. However, these polyvinyl carbazole polymers tend to become viscous and sticky when subjected to the heat-processing methods typically used to form the polymer into films or other shapes for use in photorefractive devices.
In contrast, inventors"" preferred materials, and particularly the (meth)acrylate-based, and more specifically acrylate-based, polymers, have much better thermal and mechanical properties. That is, they provide better workability during processing by injection-molding or extrusion, for example. This is particularly true when the polymers are prepared by radical polymerization.
Particular examples of monomers including a phenyl amine derivative group as the charge transport component are carbazolylpropyl(meth)acrylate monomer; 4-(N,N-diphenylamino)-phenylpropyl(meth)acrylate; N-[(meth)acroyloxypropylphenyl]-N,Nxe2x80x2, Nxe2x80x2-triphenyl-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine; N-[(meth)acroyloxypropylphenyl]-Nxe2x80x2-phenyl-N,Nxe2x80x2-di(4-methylphenyl)-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine; and N-[(meth)acroyloxypropylphenyl]-Nxe2x80x2-phenyl-N,Nxe2x80x2-di(4-buthoxyphenyl)-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine. Such monomers can be used singly or in mixtures of two or more monomers.
Particular examples of monomers including a chromophore group as the non-linear optical component are N-ethyl, N-4-dicyanomethylidenyl acrylate and N-ethyl, N-4-dicyanomethylidenyl-3, 4, 5, 6, 10-pentahydronaphtylpentyl acrylate.
Diverse polymerization techniques are known in the art. One such conventional technique is radical polymerization, which is typically carried out by using an azo-type initiator, such as AIBN (azoisobutyl nitrile). In this radical polymerization method, the polymerization catalysis is generally used in an amount of from 0.01 to 5 mol %, preferably from 0.1 to 1 mol %, per mole of the sum of the polymerizable monomers.
In the present invention, conventional radical polymerization can be carried out under inactive gas and in the presence of a solvent, such as ethyl acetate, tetrahydrofuran, butyl acetate, toluene or xylene.
Usually, the generally used inactive gas is, preferably, nitrogen, argon, or helium. Polymerization pressure is from 1 to 50 atom, preferably from 1 to 5 atom.
The solvent is generally used in an amount of from 100 to 10000 wt %, preferably from 1000 to 5000 wt %, per weight of the sum of the polymerizable monomers.
The conventional radical polymerization is preferably carried out at a temperature of from about 50xc2x0 C. to 100xc2x0 C., and is allowed to continue for about 1 to 100 hours, depending on the desired final molecular weight and polymerization temperature, and taking into account the polymerization rate.
Also, as another polymerization technique for radical polymerization, a recently developed polymerization technique known as living radical polymerization can be used for preparing polymers with unusually good photorefractive properties. In particular, living radical polymerization has the potential to form polymers with unusually low polydispersity, such as less than 2.5, such as less than 2.0. Also, particularly living radical polymerization has the capabilities to produce block copolymers, such as A-B type, A-B-A, or B-A-B type block copolymers, very efficiently.
Details of the living radical polymerization method are described in the literature. They may be founds for example, in the following papers: T. Patten et al., xe2x80x9cRadical polymerization yielding polymers with Mw/Mnxcx9c1.05 by homogeneous atom transfer radical polymerizationxe2x80x9d, Polymer Preprints, 1996, 37, 575. K. Matyjasewski et al., xe2x80x9cControlled/living radical polymerization. Halogen atom transfer radical polymerization promoted by a Cu(I)/Cu(II) redox processxe2x80x9d, Macromolecules, 1995, 28, 7901. M. Sawamoto et al., xe2x80x9cRuthenium-mediated living radical polymerization of methyl methacrylatexe2x80x9d, Macromolecules, 1996, 29, 1070.
Living radical polymerization is also described at length in U.S. Pat. No. 5,763,548 to Carnegie-Mellon University, which is incorporated herein by reference in its entirety.
Briefly, inventors"" living radical polymerization technique involves the use of a polymerization initiator, a catalyst and an activating agent.
The initiator is typically a halogen-containing organic compounds. After polymerization, this initiator or components of the initiator are attached to the polymer at both polymer terminals. The polymerization initiator preferably used is an ester-based or styrene-based derivative containing a halogen in the xcex1-position. Particularly preferred are 2-bromo(or chloro)methylpropionic acid, or bromo(or chloro)-1-phenyl derivatives. Specific examples of these derivatives include ethyl 2-bromo(or chloro)-2-methylpropionate, ethyl 2-bromo(or chloro)propionate, 2-hydroxyethyl 2-bromo(or chloro)-2-methylpropionate, 2-hydroxyethyl 2-bromo(or chloro)propionate, and 1-phenyl ethyl bromide(chloride).
Instead of a mono bromo(chloro) type initiator, a di-bromo(chloro) type initiator, such as dibromo(chloro)ester derivative, can be used. Such initiators are represented by the formula: 
wherein R0 represents a hydrogen atom or methyl group; and p is 2-6.
Of these initiators, most preferred is ethylene bis(2-bromo(chloro)-2-methylpropionate). By using this initiator, inventors have discovered that block copolymers, and particularly A-B-A type or B-A-B type block copolymers, can be produced very efficiently.
In inventors"" process, the polymerization initiator is generally used in an amount of from 0.01 to 20 mol %, preferably from 0.1 to 10 mol %, and more preferably from 0.2 to 5 mol %, per mole of the sum of the polymerizable monomers.
Various types of catalysts are known, including perfluoroalkyl iodide type, TEMPO (phenylethoxy-tetramethylpiperidine) type, and transition metal type. Inventors have discovered that high-quality polymers can be made by using transition-metal catalysts, which are safer, simpler, and more amenable to industrial-scale operation than TEMPO-type catalysts. Therefore, in the process of the invention a transition-metal catalyst is preferred.
Non-limiting examples of transition metals that may be used include Cu, Ru, Fe, Rh, V, and Ni. Particularly preferred is Cu. Typically, but not necessarily, the transition metal is used in the form of the metal halide (chloride, bromide, etc.).
The transition metal in the form of a halide or the like is generally used in the amount of from 0.01 to 3 moles, and preferably from 0.1 to 1 mole, per mole of polymerization initiator.
The activating agent is an organic ligand of the type known in the art that can be reversibly coordinated with the transition metal as a center to form a complex. The ligand preferably used is a bipyridine derivative, imercaptans derivative, trifluorate derivative, or the like. When complexed with the activating ligand, the transition metal catalyst is rendered soluble in the polymerization solvent. In other words, the activating agent serves as a co-catalyst to activate the catalyst, and start the polymerization.
The ligand is used in an amount of normally from 1 to 5 moles, and preferably from 2 to 3 moles, per mole of transition metal halide.
The use of the polymerization initiator and the activating agent in the above recommended proportions makes it possible to provide good results in terms of the reactivity of the living radical polymerization and the molecular weight and weight distribution of the resulting polymer.
In the present invention, living radical polymerization can be carried out without a solvent or in the presence of a solvent, such as butyl acetate, toluene or xylene.
To initiate the polymerization process, the monomer(s), polymerization initiator, catalyst, activating agent and solvent are introduced into the reaction vessel. As the process starts, the catalyst and polymerization initiator form a radical, which attacks the monomer and starts the polymerization growth.
The living radical polymerization is preferably carried out at a temperature of from about 70xc2x0 C. to 130xc2x0 C., and is allowed to continue for about 1 to 100 hours, depending on the desired final molecular weight and polymerization temperature, and taking into account the polymerization rate and deactivation of catalyst.
In the block copolymer preparation by living radical polymerization, the monomer addition sequence is important for achieving the desired copolymer structure. For example, to make random copolymers, both the non-linear optics-group containing and the charge transport-group-containing monomers can be added at the same time.
However, by adding the monomers sequentially, block type copolymers can be prepared. For example, to prepare an A-B type block copolymer, wherein polymer block A has charge transport ability and polymer block B has non-linear-optical ability, firstly the monomer having charge transport ability is polymerized, preferably by using a mono bromo(chloro) type initiator. Subsequently, the second monomer having non-linear-optical ability is added to continue the polymerization. In this way, an A-B type, block copolymer can be produced. During this polymerization procedure, the second monomer is added at the time when the first monomer is polymerized more than 50% by weight, normally 70% by weight or more, preferably 80% by weight or more, and more preferably 90% by weight or more.
On the other hand, if the monomer having non-linear-optical ability is polymerized first, a B-A type block copolymer can be produced. Similarly to the above polymerization procedure, the second monomer is added at the time when the first monomer is polymerized more than 50% by weight, normally 70% by weight or more, preferably 80% by weight or more, and more preferably 90% by weight or more.
Further, if living radical polymerization is carried out in a manner such that, first, the monomer having charge transport ability is polymerized, then the second monomer having non-linear-optical ability is added to continue polymerization, and thirdly an additional amount of the monomer having charge transport ability is added to continue polymerization, an A-B-A type block copolymer can be produced. During the successive polymerization procedure, the monomer to be subsequently added is added at the time when the conversion of the monomer which has been previously added exceeds at least 50% by weight, normally 60% by weight or more, preferably 80% by weight or more, and more preferably 90% by weight or more.
Moreover, if living radical polymerization is carried out in a manner such that, first, the monomer having charge transport ability is polymerized (block A), then the second monomer having non-linear-optical ability is added to continue polymerization (block B), and thirdly the monomer having plasticizer ability is added to continue polymerization (block C), an A-B-C type block copolymer can be produced. During the successive polymerization procedure, the monomer to be subsequently added is added at the time when the conversion of the monomer which has been previously added exceeds at least 50% by weight, normally 60% by weight or more, preferably 80% by weight or more, and more preferably 90% by weight or more.
If the copolymer constitutes two or more of polymer blocks A, the A-type constituting blocks need not necessarily be prepared from, the same monomer. Likewise, if the copolymer constitutes two or more of polymer blocks B, the B-type blocks need not necessarily be prepared from the same monomer. Likewise, if the copolymer constitutes two or more of polymer blocks C, the C-type blocks need not necessarily be prepared from the same monomer. Thus, the individual blocks may be of different forms represented by A1, A2, A3, etc., B1, B2; B3, and C1, C2, C3 etc. In this way, a large diversity of polymers, such as A1-B-A2, B1-B2-A, A1-B-C, B1-B2-C, or A1-B1-A2-C can be produced.
By carrying out the radical polymerization technique based on the teachings and preferences given above, it is possible to prepare random or block copolymers carrying both charge transport and non-linear optical groups. Further, by following the techniques described herein, it is possible to prepare such materials with exceptionally good properties, such as photoconductivity, response time and diffraction efficiency.
Sometimes, for better photorefractivity, the copolymer can be dispersed with a component that possesses non-linear optical properties through the polymer matrix, as is described in U.S. Pat. No. 5,064,264 to IBM, which is incorporated herein by reference. Suitable materials are known in the art and are well described in the literature, such as in D. S. Chemla and J. Zyss, xe2x80x9cNonlinear Optical Properties of Organic Molecules and Crystalsxe2x80x9d (Academic Press, 1987). Also, as described in U.S. Pat. No. 6,090,332 to Seth R. Marder et. al., fused ring bridge, ring locked chromophores that form thermally stable photorefractive compositions can be used. For typical, non-limiting examples of chromophore additives, the following chemical structure compounds can be used: 
The chosen compound(s) is sometimes mixed in the matrix copolymer in a concentration of about up to 80 wt %, more preferably 40 wt %.
On the other hand, if necessary, the copolymer can be mixed with a component that possesses charge transport properties into the polymer matrix, again as is described in U.S. Pat. No. 5,064,264 to IBM. Preferred charge transport compounds are good hole transfer compounds, for example N-alkyl carbazole or triphenylamine derivatives.
Furthermore, if necessary, the copolymer can be mixed with a component that possesses plasticizerproperties into the polymer matrix. As preferred plasticizercompounds, any commercial plasticizer compound can be used, such as phthalate derivatives or low molecular weight hole transfer compounds, for example N-alkyl carbazole or triphenylamine derivatives.
As detail examples, ethyl catbazole, 4-(N,N-diphenylamino)-phenylpropyl acatate; N-(acetoxypropylphenyl)-N,Nxe2x80x2,Nxe2x80x2-triphenyl-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine; N-(acetoxypropylphenyl)-Nxe2x80x2-phenyl-N,Nxe2x80x2-di(4-methylphenyl)-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine; and N-(acetoxypropylphenyl)-Nxe2x80x2-phenyl-N,Nxe2x80x2-di(4-buthoxyphenyl)-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine. Such compounds can be used singly or in mixtures of two or more monomers. Also, un-polymerized monomers can be low molecular weight hole transfer compounds, for example 4-(N,N-diphenylamino)-phenylpropyl(meth)acrylate; N-[(meth)acroyloxypropylphenyl]-N,Nxe2x80x2,Nxe2x80x2-triphenyl-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine; N-[(meth)acroyloxypropylphenyl]-Nxe2x80x2-phenyl-N,Nxe2x80x2-di(4-methylphenyl)-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine; and N-[(meth)acroyloxypropylphenyl]-Nxe2x80x2-phenyl-N,Nxe2x80x2-di(4-buthoxyphenyl)-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine. Such monomers can be used singly or in mixtures of two or more monomers.
To prepare the non-linear optical containing copolymer, monomers that have side-chain groups possessing non-linear-optical ability should be used. Non-limiting examples of monomers that may be used are those containing the following chemical structures: 
wherein Q represents an alkylene group with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is between about 2 and 6; R0 is a hydrogen atom or methyl group, and R is a linear or branched alkyl group with up to 10 carbons; and preferably R is a alkyl group which is selected from methyl, ethyl, and propyl.
Inventors have discovered a new technique for preparing this invention copolymers. Inventors"" technique involves the use of a precursor monomer containing a precursor functional group for non-linear optical ability. Typically, this precursor is represented by the following general formula: 
wherein R0 is a hydrogen atom or methyl group, and V is selected from the group consisting of the following structures 1 to 3: 
wherein, in both structures 1 and 2, Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is between about 2 and 6; and wherein Rd1-Rd4 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 atoms, and an aromatic group with up to 10 carbons, and preferably Rd1-Rd4 are hydrogen; and wherein R1 represents a linear or branched alkyl group with up to 10 carbons, and preferably R1 is an alkyl group selected from methyl, ethyl, propyl, butyl, pentyl and hexyls; and 
wherein Q represents an alkylene group, with or without a hetero atom, such as oxygen or sulfur, and preferably Q is an alkylene group represented by (CH2)p; where p is of about 2 to 6; and wherein R1 represents a linear or branched alkyl group with up to 10 carbons, and preferably R1 is an alkyl group selected from methyl, ethyl, propyl, butyl, pentyl and hexyls; and wherein R7 represents a linear or branched alkyl group with up to 10 carbons.
The procedure for performing the radical polymerization in this case involves the use of the same polymerization methods and operating conditions with same preferences, as have already been described above.
After the precursor copolymer has been formed, it can be converted into the corresponding copolymer having non-linear optical groups and capabilities by a condensation reaction. Typically, the condensation reagent may be selected from the group consisting of 
wherein R9, R10, R11, and R12 are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up, to 10 carbons, and an aromatic group with up to 10 carbons.
The condensation reaction can be done at room temperature for 1-100 hrs, in the presence of a pyridine derivative catalyst. A solvent, such as butyl acetate, chloroform, dichloromethylene, toluene or xylene can be used. Optionally, the reaction may be carried out without the catalyst at a solvent reflux temperature of 30xc2x0 C. or above for about 1 to 100 hours.
Inventors have discovered that use of a monomer containing a precursor group for non-linear-optical ability, and conversion of that group after polymerization tends to result in a polymer product of lower polydispersity than the case if a monomer containing the non-linear-optical group is used. This is, therefore, inventors"" preferred technique.
Also, there are no restrictions on the ratio of monomer units for the copolymers comprises a repeating unit including the first moiety having charge transport ability, a repeating unit including the second moiety having non-linear-optical ability, and a repeating unit including the third moiety having plasticizing ability. However, as a typical representative example, per 100 weight parts of [a (meth)acrylic monomer having charge transport ability], [a (meth)acrylate monomer having non-linear optical ability] is a range between 1 and 200 weight parts, preferably a range between 10 and 100 weight parts. If this ratio is less than about 1 weight part, the charge transport ability of copolymer itself is weak, and the response time tends to be too slow to give good photorefractivity. However, even in this case, by addition of already described low molecular weight components having non-linear-optical ability can enhance photorefractivity. On the other hand, if this ratio is more than about 200 weight parts, the non-linear-optical ability of copolymer itself is weak, and the diffraction efficiency tends to be too low to give good photorefractivity. However, even in this case, by addition of already described low molecular weight components having charge transport ability can enhance photorefractivity.
Also, a (meth)acrylic monomer having plasticizerability is, per 100 weight parts of a (meth)acrylic monomer having charge transport ability, is a range between 1 and 100 weight parts, preferably a range between 2 and 20 weight parts. If this ratio is more than 100 weight parts, the non-linear-optical ability and the charge transport ability of copolymer itself are weak. The response time tends to be too slow and the diffraction efficiency tends to be too low to give good photorefractivity. However, even in this case, by addition of already described low molecular weight components having non-linear-optical ability and the charge transport ability can enhance photorefractivity.
Optionally, other components may be added to the polymer matrix to provide or improve the desired physical properties mentioned earlier in this section. Usually, for good photorefractive capability, it is preferred to add a photosensitizer to serve as a charge generator. A wide choice of such photosensitizers is known in the art. Typical, but non-limiting examples of photosensitizers that may be used are 2, 4,7-trinitro-9-fluorenone (TNF) and C60. The amount of photosensitizer required is usually less than 3 wt %.
As mentioned above, it is preferred that the copolymer matrix have a relatively low glass-transition temperature, and be workable by conventional processing techniques. Optionally, a plasticizer may be added to the composition to reduce the glass transition temperature and/or facilitate workability. The type of plasticizer suitable for use in the invention is not restricted; many such materials will be familiar to those of skill in the art. Representative typical examples include N-alkylcarbazole and dioctylphthalate. Oligomer-type compounds of the charge transport or non-linear-optical monomers may also be used to control the Tg of the composition.
In general, the smallest amount of plasticizer required to provide a suitable overall Tg for the composition should be used. Compositions with large amounts of plasticizer tend to have lower stability, as the polymer matrix and the plasticizer may phase separate over time. Also, the photorefractive properties of the material are diminished by dilution of the active components by the plasticizer.
As discussed above, the invention provides polymers of comparatively low Tg when compared with similar polymers prepared in accordance with prior art methods. Inventors have recognized that this provides a benefit in terms of lower dependence on plasticizers. By selecting copolymers of intrinsically moderate Tg and by using methods that tend to depress the average Tg, it is possible to limit the amount of plasticizer required for the composition to preferably no more than about 30% or 25%, and more preferably lower, such as no more than about 20%.
In the art, many of the compositions of the photorefractive polymers showed poor phase stabilities and gave haziness after days. Once the composition films showed the haziness, they don""t show good photorefractive properties. This film composition haziness is usually coming from incompatibilities between several photorefractive components. Generally, photorefractive compositions comprise from components having charge transport ability and components having non-linear optics ability. The components having charge transport ability are usually hydrophobic and nonpolar material. On the other hand, components having non-linear optics ability are usually hydro-philic and polar material. Therefore, as a nature of these components, there were tendencies to be phase separated and give hazy compositions.
In the previously described paper (Macromolecules, 2000, 33, 4074), acrylate-based polymers that include carbazole-based side chains and several stilibene-type side chains comprise of components having charge transport ability and the components having non-linear optics ability. In this paper, it is said these polymers can be expected to have good phase stability, although there is no actual detail data.
However, on the other hand, in the present invention, the photorefractive polymers composition showed very good phase stabilities and gave no haziness even after several months. They don""t change good photorefractive properties, as the composition are very stable and no phase separations are observed. These film composition stabilities are probably due to copolymer structures. That is, the components having charge transport ability and the components having non-linear optics ability are existing in one polymer chain and phase separation are irrelevant and unlikely to happen.
This good phase stabilities of this invention can be obtained even by addition of charge transport, non-linear optical, or plasticizer low molecular weight compounds. Seemingly, this invention copolymer gave more tolerance for phase stability, since the copolymer can work as a kind of phase compatibilizer.
This good phase stabilities of this invention last more than a day or a week, or sometimes more than six months. Also, even by heating up the testing samples, which usually enhance phase separation speed, the samples showed very good phase stability for more than a day or a week, or sometimes more than six months. These good phase stability can facilitate the invention copolymer into optical device applications for more commercial products. For acceleration tests, heating test temperature have no restriction, but usually, the temperature is between 40 and 120xc2x0 C., preferably between 60 and 80xc2x0 C.
The photorefractive materials of the invention provide combinations of desirable properties not previously available to the art.
One particularly advantageous feature is the high photoconductivity. In the context of the invention, by photoconductivity inventors mean the increase in conductivity of the photorefractive material under laser irradiation. The photoconductivity of a sample of material may be measured by the following method. First, the steady-state conductivity properties are measured in the dark, by applying an electric field across the sample, allowing the system to come to steady state, and measuring the resulting current. Then the measurements are repeated while illuminating the sample with a pulse from a single laser beam. The photoconductivity, "PHgr"photo, can then be calculated using the following relation:
"PHgr"photo=(i photoxe2x88x921dark)/AbeamEa
where:
Ea is the applied electric field,
Abeam is the illuminated area,
iphoto is electric current with laser irradiation, and
idark is electric current without irradiation
Photoconductivity is important because it is a measure of how efficiently charge transport can take place in the material. If all other parameters are fixed, the higher the photoconductivity, the faster is the device response time.
Typical photoconductivities for organic materials and polymers are in the range from about 0.01 pS/cm to a maximum of no higher than 1,000 pS/cm. In fact, the value for conventional photorefractive polymers is usually less than 10 pS/cm, as described in several papers, for example in M. A. Diaz-Garcia et al. (Chem. Mater., 1999, 11, 1784). However, surprisingly, the polymers of the invention generally have photoconductivity of more than 100 pS/cm, which is at least an order of magnitude better than currently used materials, and preferably exhibit a photoconductivity of about 200 pS/cm or more, and more preferably about 500 pS/cm or more.
Very few materials have been reported with photoconductivity higher than 10 pS/cm, and even fewer with photoconductivity higher than 100 pS/cm. To inventors"" knowledge, the highest photoconductivity ever reported is 2,800 pS/cm (N. Peyghambarian et al., J. Mater. Chem., 1999, 9, 2251), in a PVK-type material.
Yet, for inventors"" materials inventors have measured photoconductivity of 3,000 pS/cm or more, specifically 3,500 pS/cm.
Furthermore, inventors are aware of no photorefractive composition that provides a photoconductivity of at least 100 pS/cm and, at the same time, offers any one of the other advantageous properties provided herein, such as a polydispersity of no more than about 2.5 or 2.0, a response time of no more than about 50 ms, a diffraction efficiency of at least about 5%, the good mechanical properties and easy processability of an acrylate-based polymer, the presence of both charge transport and non-linear optical groups as side chains in the polymer, especially in the form of block copolymers, or the efficiency and flexibility of preparation of the living radical polymerization technique.
Another particularly advantageous feature is the fast response time. Response time is the time needed to build up the diffraction grating in the photorefractive material when exposed to a laser writing beam. The response time of a sample of material may be measured by transient four-wave mixing (TFWM) experiments, as detailed in the Examples section below. The data may then be fitted with the following bi-exponential function:
xcex7(t)=xcex70(1xe2x88x92a1exe2x88x92t/J1xe2x88x92a2exe2x88x92t/J2)2
with a1+a2=1
where xcex7(t) is the diffraction efficiency at time t, xcex70 is the steady-state diffraction efficiency, and J1 and J2 are the grating build-up times. The smaller number of J1 and J2 is defined as the response time.
Response time is important because a faster response time means faster grating build-up, which enables the photorefractive composition to be used for wider applications, such as real-time hologram applications.
Typical response times for known photorefractive materials range from seconds to sub-seconds. Times longer than 100 ms are common. Faster response times have been reported, see W. F. Moemer, Appl. Phys. Lett., Vol. 73, p. 1490 (1998) but, in order to get these higher speeds, higher field strengths have been required. Such higher field strengthsmay be difficult in an industrial, rather than a laboratory, environment. Also, the polyvinyl carbazole polymersused to obtain higher speeds become sticky and difficult to handle during heat processing. In contrast, the acrylate-based, or more specifically methacrylate-based polymers, that are preferred herein provide excellent workability during heat processing and other polymer handling methods.
In comparison with typical prior art materials, the photorefractive compositions of the invention provide good response times, such as no more than about 50 ms, and preferably faster, such as no more than about 40 ms, no more than about 30 ms, or no more than about 20 ms.
Furthermore, these response times can be achieved without resorting to a very high electric field, expressed as biased voltage. By a very high biased voltage, inventors mean a field in excess of about 100V/xcexcm. In inventors"" materials, fast response times can generally be achieved at biased voltages no higher than about 100 V/xcexcm, more preferably no higher than about 90 V/xcexcm.
And, as discussed with respect to photoconductivity, these good response times can be provided in conjunction with one or more of the other advantageous properties as they are characterized above, such as high photoconductivity, low polydispersity, high diffraction efficiency, good processing capabilities, block copolymer capability, and efficient polymerization techniques.
Yet another advantageous feature is the diffraction efficiency, xcex7. Diffraction efficiency is defined as the ratio of the intensity of the diffracted beam to the intensity of the incident probe beam, and is determined by measuring the intensities of the respective beams. Obviously, the closer to 100% is the ratio, the more efficient is the device.
In general, for a given photorefractive composition, a higher diffraction efficiency can be achieved by increasing the applied biased voltage.
In comparison with typical prior art materials, the photorefractive compositions of the invention provide good diffraction efficiencies, such as at least about 5%, and preferably higher, such as at least about 10%. And, as discussed with respect to photoconductivity, these good diffraction efficiencies can be provided in conjunction with one or more of the other advantageous properties as they are characterized above, such as high photoconductivity, or fast response time, and in conjunction with good processing capabilities, block copolymer capability, and efficient polymerization techniques.
The invention is now further described by the following examples, which are intended to be illustrative of the invention, but are not intended to limit the scope or underlying principles in any way.